Re: [boost] [lockfree::fifo] Review

Subject: Re: [boost] [lockfree::fifo] Review
From: Helge Bahmann (hcb_at_[hidden])
Date: 2009-12-11 10:35:08

On Fri, 11 Dec 2009, Andy Venikov wrote:

> Helge Bahmann wrote:
>> This is however NOT possible because you are not allowed to make any memory 
>> access between ll and sc -- some CPUs (definitely older mips, but not ppc, 
>> unsure about many of the other architectures) clear the reservation on 
>> _any_ memory reference and will cause this sequence to always fail. Alpha 
>> has the fun requirement of a memory barrier before you are allowed to 
>> dereference "tmp", which just provides another way to kill your 
>> reservation.
> PPC as well. 7447, for example, will invalidate the "reservation bit" after 
> writing to ANY memory location.

Cool, this makes the whole thing model-specific behavior and thus even 
more fun :) On the ppc 7455 available to me it not only works, but the 
"stwcx." in fact behaves more like a CAS (it remembers the value loaded by 
lwarx, and succeeds if it matches -- thus ABA detection won't work for 
entirely different reasons).

Other fun restrictions that I remember from some architecture manual (I 
don't know which, think it was also some mips flavor): No taken branch 
allowed between ll/sc.

The message should however be clear: ll/sc is *brittle* and not usable for 
ABA prevention.

Helge